Semiconductor device and manufacturing method for the same

ABSTRACT

A semiconductor device having an active layer comprising crystalline silicon, said active layer comprising a first layer comprising crystalline silicon formed on an insulating surface and a second layer comprising crystalline silicon formed on said first layer, wherein said first layer contains a metal element at a first concentration while said second layer is free from said metal element or contains said metal element at a second concentration which is lower than said first concentration.

This application is a Divisional of application Ser. No. 08/922,381,filed Sep. 3, 1997; now U.S. Pat. No. 5,773,847; which itself is acontinuation of Ser. No. 08/452,705, filed May 30, 1995, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device in which asemiconductor having crystallinity is used, and to a manufacturingmethod for the same.

BACKGROUND OF THE INVENTION

A thin film transistor (TFT) is a known device in which a thin filmsemiconductor is used. A TFT is composed by forming a thin filmsemiconductor on a substrate and using this thin film semiconductor forthe semiconductor regions of a transistor. TFTs are used for variousintegrated circuits, and particularly for active matrix type liquidcrystal display units and the like.

As the thin film semiconductor in a TFT, it is convenient to use anamorphous silicon film but there is the problem that they have poorelectrical characteristics. Accordingly, a crystalline silicon film isutilized in order to improve the characteristics of a TFT. To obtainthis crystalline silicon film, an amorphous silicon film is formed andthen crystallized by a heat treatment.

However, the crystallization by heating has required a high temperatureprocess at 600° C. or higher, and in addition, it has been necessary totake 10 hours or longer. This has caused the problem that it isdifficult to use an inexpensive glass substrate having a low distortionpoint.

Research carried out by the present inventors has shown that theaddition of trace amounts of elements such as nickel, palladium and leadto an amorphous silicon film makes it possible to carry outcrystallization by heat treatment of about 4 hours in a low temperatureprocess of 550° C. or lower. Further, it has become clear that also whenthe crystallization is carried out by laser, the same effect can beobtained.

However, the presence large quantities of impurities such as nickel in asemiconductor damage the device characteristics and reliability of adevice using the semiconductor, and is undesirable. That is, elementssuch as nickel needed in crystallizing an amorphous silicon, arerequired not to be contained as far as possible in the crystallinesilicon obtained.

Further, when the crystallization is carried out by laser, ridges of thecrystal growth on a crystal surface in the form of projections areformed. Since the ridges have an influence on the flatness of a filmsurface, it is desirable that as far as possible they do not exist.

SUMMARY OF THE INVENTION

An object of the present invention is to form a thin film semiconductorlayer having a very low content of elements such as nickel by employingheat crystallization in a low temperature process.

The present invention is characterized in that an amorphous silicon filmis divided into two layers and a heat crystallization process is carriedout twice to thereby easily obtain a crystalline silicon film having avery low content of elements such as nickel by a low temperatureprocess.

In the present invention, since the most remarkable effect can beobtained when nickel is used, nickel will be used in the followingdescription. However, other usable elements include Pt, Cu, Ag, Au, In,Sn, Pd, P, As, and Sb. Further, there can be used as well one or moreelements selected from the VIII group elements and the IIIb, IVb and Vbgroup elements.

First, nickel which promotes the crystallization of an amorphous siliconfilm, is introduced into the amorphous silicon film constituting thefirst layer, and a heat treatment is carried out to crystallize theamorphous silicon film. Then, an amorphous silicon film constituting thesecond layer is formed, and heat treatment is carried out to crystallizethis amorphous silicon film. In this way, a crystalline silicon filmsuitable for a thin film semiconductor layer is obtained.

For the introducing method for the nickel element promoting thecrystallization, a layer containing nickel or a nickel compound can beformed in contact with the amorphous silicon film. For forming thenickel-containing layer, a method in which a solution containing nickelis coated and then dried (for example, spin coating or dipping), amethod in which a nickel or a nickel compound film is formed bysputtering, or a method in which gaseous organic nickel is used as aprecursor material of a vapor phase deposition using heat, light and/orplasma as an energy source. In all the methods, the thickness of thelayer may be determined according to the amount of nickel needed.

When the nickel-containing layer is deposited by sputtering, nickelsilicide may be used as a material for a sputtering target, besidesnickel.

With respect to the methods involving coating and drying a solutionamong the methods for forming the nickel-containing layer, an aqueoussolution or an organic solvent solution can be used as the solution.Here, the term "containing" includes both the meaning of containing as acompound and the meaning of containing as a dispersion.

When a solvent selected from water, alcohol, acid and ammonia, which arepolar solvents, is used as the solvent, the nickel compound constitutingthe solute are typically selected from among nickel bromide, nickelacetate, nickel oxalate, nickel carbonate, nickel iodide, nickelnitrate, nickel sulfate, nickel formate, nickel acetylacetonate, nickel4-cyclohexylbutyrate, nickel oxide, and nickel hydroxide.

When a solvent selected from among benzene, toluene, xylene, carbontetrachloride, chloroform, and ether, which are non-polar solvents, isused, the nickel compound selected from among nickel acetylacetonate andnickel 2-ethylhexanoate can be typically used. Naturally, other solventsand solutes may be used.

It is also useful to add a surfactant to the solution containing thecatalyst element. This is to enhance adhesion to the surface to becoated and to control absorption of the solution. This surfactant may becoated in advance on the surface to be coated.

When an elemental nickel is used as the catalyst element, it must bedissolved in an acid to make a solution.

What was described above is an example that a solution in which thecatalyst element nickel is completely dissolved is used; however, amaterial like an emulsion in which a powder comprising a nickel elementor a nickel compound is evenly dispersed in a dispersion medium withoutthe nickel being completely dissolved. Further, a solution for formingan oxide film may be used. Such solutions include OCD (Ohka DiffusionSource) produced by Tokyo Ohka Kogyo Co., Ltd. The use of this OCDsolution makes it possible to readily form a silicon oxide film bycoating it on the surface on which the film is to be formed and bakingat about 200° C. Nickel can be diffused in an amorphous silicon film byincorporating nickel into this silicon oxide film.

When a polar solvent like water is used as the solvent, the solution isrepelled when the solution is coated directly onto the amorphous siliconfilm. In this case, a thin oxide film of 100 Å or less is first formed,and a solution containing a catalyst element is coated thereon, wherebythe solution can be coated evenly. Also methods for improving wetting byadding a material like a surfactant to the solution are effective aswell.

Direct coating on the amorphous silicon film can be carried out using anon-polar solvent like a toluene solution of nickel 2-ethylhexanoate asthe solution. In this case, it is effective and preferable to coat inadvance a material like an adhesive used in coating a resist. However,attention has to be paid so that too much is not coated, otherwise theaddition of the catalyst element to the amorphous silicon will beactually be obstructed.

The amount of nickel contained in the solution depends on the type ofthe solution. As a general standard, the nickel amount in the solutionshould be 200 ppm to 1 ppm, preferably 100 ppm to 1 ppm (by weight).This is a value determined in view of the nickel concentration andhydrofluoric acid resistance of the film after the crystallization.

A nickel-containing layer is formed in contact with an amorphous siliconfilm constituting the first layer and then heat treated at 450 to 550°C. for 4 to 8 hours to effect thermal crystallization. The crystallinesilicon film thus obtained contains trace amounts of nickel asimpurities.

Then, an amorphous silicon film constituting the second layer is formedon the crystalline silicon film. It is subjected to the same heattreatment as the first layer to obtain a crystalline silicon film. Inthe crystal growth of the amorphous silicon film constituting the secondlayer, the parts contacting the surfaces of the crystalline silicon filmconstituting the first layer are crystallized in succession with thecrystalline structure of the surface of the crystalline silicon filmconstituting the first layer as a nucleus for the crystal growth. Sinceno nickel is introduced into this amorphous silicon film constitutingthe second layer, impurities are not substantially contained, and acrystalline silicon film suitable as a semiconductor layer is obtained.

A crystalline silicon film containing substantially no impurities can beobtained in a low temperature process by forming the crystalline siliconfilm in two layers separately as in the present invention.

When the two-layer crystalline silicon film thus obtained is used as athin film semiconductor layer in a TFT, since a channel is formedsubstantially to a depth of about 200 to 300 Å, the crystalline siliconfilm formed as the second layer constitutes a substantially activelayer.

That is, since the crystalline silicon film constituting the secondlayer containing substantially no nickel is used as a semiconductorlayer of a TFT, good device characteristics and reliability can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) to 1(D) show the process of Preferred Embodiment 1;

FIGS. 2(A) to 2(D) show the process of Preferred Embodiment 2;

FIGS. 3(A) to 3(F) show the process of Preferred Embodiment 3; and

FIGS. 4(A) to 4(F) show the process of Preferred Embodiment 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred Embodiment 1

The present embodiment is shown in FIGS. 1(A) to 1(D). The presentpreferred embodiment is an example in which nickel for promotingcrystallization is introduced into an amorphous silicon film and thencrystallization is carried out by a thermal crystallization. Anamorphous silicon film is then further formed on this crystallinesilicon film and crystallized by thermal crystallization.

First, silicon oxide film 102 was formed to 1000 to 5000 Å, for example4000 Å on a substrate 101 (Corning 7059, 100 mm×100 mm) as a base oxidefilm by sputtering. This silicon oxide film 102 is provided in order toprevent impurities from diffusing from the glass substrate. Then, anamorphous silicon film 103 was formed to 300 to 1500 Å by plasma CVD orLPCVD. Here, the amorphous silicon film 103 was formed to a thickness of500 Å by plasma CVD. (FIG. 1(A))

Thereafter, a layer 104 (nickel-containing layer) containing nickel or anickel compound of several to several tens of Å was formed on theamorphous silicon film 103. The nickel-containing layer can be formed bya method in which a solution containing nickel is coated and then dried(for example, spin coating or a dipping), a method in which a nickel ora nickel compound film is formed by sputtering, or a method in which agaseous organic nickel is decomposed and deposited by a vapor phasemethod using heat, light and/or plasma as an energy source. Here, thefilm was formed by a spin coating method. (FIG. 1(B))

First, an oxide film is formed to 10 to 50 Å on the amorphous siliconfilm 103 by irradiation with UV rays in an oxygen atmosphere, thermaloxidation or treatment with hydrogen peroxide. Here, the oxide film wasformed to 20 Å by irradiation with UV rays in an oxygen atmosphere. Thisoxide film is for spreading an acetate solution containing nickel overthe whole surface of the amorphous silicon film in the following processof coating the acetate solution; that is, it is to improve wetting.

Next, a solution was prepared by adding nickel to an acetate solution.The concentration of nickel was set to 25 ppm. Then, 2 ml of thisacetate solution was dropped onto the surface of the substrate while thesubstrate was rotated, and this state was maintained for 5 minutes tospread the nickel acetate solution evenly over the substrate. Then, thespeed was raised to carry out spin drying (2000 rpm, 60 seconds).

The concentration of nickel in the acetate solution is 1 ppm or more forpractical use. A nickel acetate layer 104 having an average thickness of20 Å could be formed on the surface of the amorphous silicon film afterthe spin drying by carrying out this coating process of the nickelsolution once or several times. The layer can be formed similarly usingother nickel compounds. The inventors of the present invention confirmedthat the nickel compound layer was substantially homogenous.

In the present embodiment, a method by which nickel or a nickel compoundis introduced on the amorphous silicon film was shown, but a method bywhich nickel or a nickel compounds is introduced under the amorphoussilicon film may alternatively be employed. In this case, the nickel ornickel compounds is introduced before forming the amorphous siliconfilm.

Heat treatment was then carried out at 550° C. for 4 hours in a nitrogenatmosphere in a heating furnace. This produced a crystalline siliconfilm 105 constituting a first layer on the substrate. (FIG. 1(C))

An amorphous silicon film was formed to 200 to 800 Å, for example 500 Åon this crystalline silicon film 105 by plasma CVD. Then, heat treatmentwas carried out again at 550° C. for 4 hours in a nitrogen atmosphere ina heating furnace. This resulted in a crystalline silicon film 106constituting a second layer being obtained on the crystalline siliconfilm 105 constituting the first layer. (FIG. 1(D))

In this case, the nickel which was added in carrying out thecrystallization is present as impurities in the crystalline silicon film105 constituting the first layer, but no impurities are contained in thecrystalline silicon film 106 constituting the second layer, andtherefore a semiconductor layer having good device characteristics canbe obtained. In the crystal growth of the crystalline silicon film 106constituting the second layer, crystal growth reflecting the crystalstructure of the crystalline silicon film 105 constituting the firstlayer therebelow can be observed. Accordingly, since the crystallinesilicon film 105 constituting the first layer grew longitudinally,corresponding growth has observed in the crystalline silicon film 106constituting the second layer also.

Preferred Embodiment 2

The present embodiment is an example in which a silicon oxide film ofthickness 1200 Å is provided selectively on an amorphous silicon film,and nickel is selectively introduced with this silicon oxide film beingused as a mask to crystallize the amorphous silicon film. The siliconoxide film is then etched and a crystalline silicon film constituting asecond layer is obtained in the same way as in preferred embodiment 1.

An outline of the production process of this preferred embodiment isshown in FIGS. 2(A) to 2(D). First, a silicon oxide film 202 was formedto 5000 Å as a base oxide film on a substrate 201 by depositing TEOS byplasma CVD. An amorphous silicon film 203 was then formed to a thicknessof 500 Å by plasma CVD.

Then, a silicon oxide film 204 constituting a mask was formed on theamorphous silicon film 203 to a thickness of 1000 Å or more, here 1200Å. The silicon oxide film 204 was patterned to a required pattern by aconventional photolithography patterning process. (FIG. 2(A))

Then, a nickel-containing layer 205 of several to several tens of Å wasformed on the amorphous silicon film 203. Here, a nickel layer 205having an average thickness of 20 Å was formed by sputtering. This layerdoes not necessarily form a complete film. (FIG. 2(B))

The thermal crystallization was then carried out. Here, heat treatmentwas carried out at 550° C. (nitrogen atmosphere) for 8 hours tocrystallize the amorphous silicon film 203. In this case, nickel wasintroduced from an aperture part formed by the patterning of the siliconoxide film 204, and crystal growth proceeded in a lateral direction fromthe region into which this nickel had been introduced to the region intowhich the nickel had not been introduced.

Then, the nickel-containing layer 205 remaining on the crystallinesilicon film 206 was removed with a chlorine etchant. The silicon oxidefilm 204 which had been used as a mask was removed with bufferedhydrofluoric acid. (FIG. 2(C))

An amorphous silicon film was then formed to 200 to 800 Å, for example300 Å on the crystalline silicon film 206 obtained in the processdescribed above by plasma CVD. Then, thermal crystallization was carriedout again at 550° C. for 8 hours in a heating furnace in a nitrogenatmosphere. As a result, a crystalline silicon film 207 constituting asecond layer has obtained on the crystalline silicon film 206constituting a first layer. In this case, trace amounts of nickel werecontained as impurities in the crystalline silicon film 206 constitutingthe first layer as is the case in the Preferred Embodiment 1, butimpurities were not contained in the crystalline silicon film 207constituting the second layer. Since the crystalline silicon film 206constituting the first layer grew laterally, in the crystalline siliconfilm 207 constituting the second layer also crystal growth reflectingthe crystalline structure of the lower layer was observed.

Preferred Embodiment 3

This preferred embodiment is an example in which a crystalline siliconfilm formed utilizing the method of the present invention is used toobtain a TFT.

An outline of the production process of the present preferred embodimentis shown in FIGS. 3(A) to 3(F). First, a base silicon oxide film 302 wasformed to a thickness of 2000 Å on a substrate 301. Then, an amorphoussilicon film was formed to a thickness of 500 Å by plasma CVD. Aftertreating with hydrofluoric acid to remove a natural oxide film, a thinoxide film was formed to a thickness of about 20 Å by irradiation withUV rays in an oxygen atmosphere.

An acetate solution containing 10 ppm nickel was coated and held for 5minutes, after which spin drying has carried out using a spinner. Then,heat was applied at 550° C. for 4 hours in a nitrogen atmosphere tocrystallize the silicon film into a crystalline silicon film 303. (FIG.3(A))

The nickel-containing layer remaining on the crystalline silicon film303 was then removed by etching with a chlorine etchant. Here, theetching may be carried out leaving about 200 Å of the crystallinesilicon film 303 remaining.

An amorphous silicon film was formed to 400 Å on this crystallinesilicon film 303 by plasma CVD. Then, heat treatment was carried outagain at 550° C. for 4 hours in a heating furnace in a nitrogenatmosphere. As a result, a crystalline silicon film 304 constituting asecond layer has obtained on the crystalline silicon film 303constituting a first layer. (FIG. 3(B))

Next, the crystalline silicon films of these two layers were patternedto form an island region 305. This island region 305 constitutes anactive layer of a TFT. Silicon oxide having a thickness of 200 to 1500Å, here 1000 Å was formed as a gate insulating film 306 by plasma CVD.

Then, an aluminum (containing Si, 1 wt % or Sc, 0.1 to 0.3 wt %) filmhaving a thickness of 1000 Å to 3 μm, for example 5000 Å was formed bysputtering, and this was patterned to form a gate electrode 307. Next,the substrate was dipped in an ethylene glycol solution containing 1-3%tartaric acid and of a pH approximately 7, and anodic oxidation wascarried out with platinum as a cathode and this aluminum gate electrode307 as an anode. The anodic oxidation was finished after initiallyraising the voltage up to 220 V at a fixed current and maintaining thevoltage at 220 V for one hour. Thus, an anodic oxide having a thicknessof 1500 to 3500 Å, for example 2000 Å was formed. (FIG. 3(C))

Then, an impurity (phosphorus) was injected into the silicon film by iondoping with the gate electrode 307 used as a mask. Phosphine gas (PH₃)was used as the doping gas. In this case, the dose amount was 1×10¹⁴ to5×10¹⁷ cm⁻², and the accelerating voltage was 10 to 90 kV, for examplethe dose amount being set to 2×10¹⁵ cm⁻² and the accelerating voltage to80 kV. This resulted in an N type impurity region 308 being formed(source/drain region). (FIG. 3(D))

A KrF excimer laser (wavelength: 248 nm, pulse width: 20 nsec) was usedto irradiate and activate the impurity region 308. A suitable energydensity of the laser was 200 to 400 mJ/cm², and preferably 250 to 300mJ/cm². This process may be carried out by the heat annealing.

Next, a silicon oxide film 309 was formed as an interlayer insulatingfilm 309 to a thickness of 3000 Å by plasma CVD. In this case, TEOS andoxygen were used for the feed gas. (FIG. 3(E))

The interlayer insulating film 309 and the gate insulating film 306 werethen etched to form contact holes to the source and the drain. Then, analuminum film was formed by sputtering and then patterned to form sourceand drain electrodes 310, whereby a TFT was produced. (FIG. 3(F))

After forming the TFT, hydrogenation treatment may further be carriedout at 200 to 400° C. for activation of the impurity region.

Preferred Embodiment 4

This preferred embodiment is an example in which a crystalline siliconfilm formed by utilizing the method of the present invention is used toobtain a CMOS type TFT.

An outline of the production process of this preferred embodiment isshown in FIGS. 4(A) to 4(D). First, a base silicon oxide film 402 wasformed to a thickness of 3000 Å on a substrate 401. An amorphous siliconfilm was then formed to a thickness of 500 Å by plasma CVD. Then, asilicon oxide film constituting a mask was formed on the amorphoussilicon film to a thickness of 1200 Å. The silicon oxide film waspatterned to a required pattern by a conventional photolithographypatterning process to form an aperture part from which nickel would beintroduced. A thin oxide film was formed to a thickness of about 20 Å byirradiation with UV rays in an oxygen atmosphere.

An acetate solution containing 50 ppm of nickel was coated and held for5 minutes, after which spin drying was carried out using a spinner.Then, heat was applied at 550° C. for 8 hours in a nitrogen atmosphereto crystallize the silicon film into a crystalline silicon film 403.(FIG. 4(A))

After that,the nickel-containing layer remaining on the crystallinesilicon film 403 was removed by etching with a chlorine etchant. Also,the silicon oxide film which had been used as the mask was removed withbuffered hydrofluoric acid. Here, etching may be carried out leavingabout 200 Å of the crystalline silicon film 403 remaining.

An amorphous silicon film was formed to 300 Å on this crystallinesilicon film 403 by plasma CVD. Then, heat treatment was carried outagain at 550° C. for 8 hours in a heating furnace in a nitrogenatmosphere. This produced a crystalline silicon film 404 constituting asecond layer on the crystalline silicon film 403 constituting a firstlayer. (FIG. 4(B)).

Next, the crystallized silicon films were patterned to form an islandregion. This island region constitutes an active layer of a TFT. Siliconoxide having a thickness of 200 to 1500 Å, here 1000 Å was formed as agate insulating film 405 by plasma CVD.

After that, an aluminum (containing Si, 1 wt % or Sc, 0.1 to 0.3 wt %)film having a thickness of 1000 Å to 3 μm, for example 5000 Å was formedby sputtering, and this was patterned to form gate electrodes 406 and407. Next the substrate was dipped in a 1 to 3% solution of tartaricacid in ethylene glycol, of pH about 7, and anodic oxidation was carriedout with platinum as a cathode and these aluminum gate electrodes 406and 407 as anodes. The anodic oxidation was finished after initiallyraising the voltage up to 220 V at a fixed current and then maintainingthe voltage at 220 V for one hour. Thus, an anodic oxide having athickness of 1500 to 3500 Å, for example 2000 Å was formed.

Then, impurities were injected into the insular silicon film by iondoping with the gate electrodes 406 and 407 used as a mask. Here,phosphorus has used as an N type impurity and boron as a P typeimpurity. First, phosphorus was injected into the whole surface. In thiscase, the dose amount was 1×10¹⁴ to 5×10¹⁷ cm⁻², and the acceleratingvoltage was 10 to 90 kV, for example the dose amount being set to 1×10¹⁵cm⁻² and the accelerating voltage to 80 kV. As a result, N type impurityregions 408 and 409 were formed. (FIG. 4(C))

Next, boron was injected with the N channel type TFT region covered witha photoresist 410. In this case, the dose amount was several to severaltens of times as much as for the N type impurity region, here 4×10¹⁵cm⁻², and the accelerating voltage was set to 65 kV. As a result, thepart which had been the N type impurity region 409 has inverted and a Ptype impurity region 411 has formed. (FIG. 4(B)).

The doped impurity regions 408 and 411 were activated by irradiationwith a KrF excimer laser (wavelength: 248 nm, pulse duration: 20 nsec).A suitable energy density of the laser was 200 to 400 mJ/cm², andpreferably 250 to 300 mJ/cm². This process may be carried out by theheat annealing.

Next, a silicon oxide film 412 formed as an interlayer insulating film412 to a thickness of 3000 Å by plasma CVD. (FIG. 4(E))

The interlayer insulating film 412 and the gate insulating film 405 werethen etched to form contact holes to the source and the drain. Then, analuminum film was formed by sputtering and patterned to formsource/drain electrodes 413, 414 and 415. A CMOS type TFT was producedby the processes described above. (FIG. 4(F)).

According to the present invention, crystalline silicon films having fewimpurities can be formed at lower temperatures than in the past byforming a crystalline silicon film in two layers. A device having goodcharacteristics can be obtained by making a semiconductor device usingcrystalline silicon films thus obtained.

While the preferred embodiments of the present invention are describedabove, the present invention should not be limited to these examples.

What is claimed is:
 1. A method of manufacturing a semiconductor devicecomprising the steps of:forming a first semiconductor film comprisingamorphous silicon on an insulating surface; forming a catalyst materialin contact with said first semiconductor film comprising amorphoussilicon, said catalyst material being capable of promotingcrystallization of silicon; crystallizing said first semiconductor filmby heating; forming a second semiconductor film comprising amorphoussilicon on said crystallized first semiconductor film; and thencrystallizing said second semiconductor film by heating.
 2. A methodaccording to claim 1 wherein said catalyst material is formed in contactwith a selected portion of said semiconductor film.
 3. A methodaccording to claim 1 wherein said catalyst material comprises at leastan element selected from the group consisting of Ni, Pt, Cu, Ag, Au, In,Sn, Pd, P, As, and Sb.
 4. A method according to claim 1 furthercomprising the step of forming a gate electrode over said secondsemiconductor film with a gate insulating film therebetween.
 5. A methodaccording to claim 1 wherein said catalyst material comprises at leastan element selected from the group consisting of VIII, IIIb, IVb, and Vbgroups in the periodic table.
 6. A method according to claim 1 whereinsaid catalyst material is formed through a method selected from thegroup consisting of spin coating, dipping, sputtering, vapor phasedeposition methods.
 7. A method according to claim 1 wherein each ofsaid first and second semiconductor film is a silicon film.
 8. A methodaccording to claim 1 further comprising the steps of:patterning saidcrystallized first and second semiconductor films to form a crystallinesemiconductor island; forming a gate insulating film in contact withsaid crystalline semiconductor island; forming a gate electrode portionadjacent to said crystalline semiconductor island having said gateinsulating film therebetween; introducing an impurity to give oneconductivity type into said crystalline semiconductor island using saidgate electrode portion as a mask so that source and drain regions, and achannel region therebetween are formed in said crystalline semiconductorisland.
 9. A method of fabricating a semiconductor device comprising thesteps of:forming a first semiconductor film comprising amorphous siliconon an insulating surface; selectively forming a catalyst material incontact with a first portion of said first semiconductor film while saidcatalyst material is not formed in contact with a second portion of saidfirst semiconductor film, said catalyst material being capable ofpromoting crystallization of said first semiconductor film; heating saidfirst semiconductor film so that crystal growth proceeds in a lateraldirection to said insulating surface from said first portion to saidsecond portion; forming a second semiconductor film comprising amorphoussilicon on said first crystallized semiconductor film; heating saidsecond semiconductor film to crystallize.
 10. A method according toclaim 9 further comprising the steps of:patterning said crystallizedfirst and second semiconductor films to form a crystalline semiconductorisland; forming a gate insulating film in contact with saidsemiconductor island; forming a gate electrode portion adjacent to saidcrystalline semiconductor island having said gate insulating filmtherebetween; introducing an impurity to give one conductivity type intosaid crystalline semiconductor island using said gate electrode portionas a mask so that source and drain regions, and a channel regiontherebetween are formed in said crystalline semiconductor island.
 11. Amethod according to claim 9 further comprising the steps of:patterningsaid crystallized first and second semiconductor films to form first andsecond crystalline semiconductor islands; forming a gate insulating filmin contact with said first and second crystalline semiconductor islands;forming a first gate electrode portion adjacent to said firstcrystalline semiconductor island having said gate insulating filmtherebetween and a second gate electrode portion adjacent to said secondcrystalline semiconductor island having said gate insulating filmtherebetween; introducing an n-type impurity at a first concentrationinto said first and second crystalline semiconductor islands using saidfirst and second gate electrode portions as masks so that first sourceand drain regions, and a first channel region therebetween are formed insaid first crystalline semiconductor island; covering said firstcrystalline semiconductor island by a photoresist; introducing a p-typeimpurity at a second concentration into only said second crystallinesemiconductor island using said second gate electrode portion as a maskso that second source and drain regions, and a second channel regiontherebetween are formed in said second crystalline semiconductor island,said second concentration being higher than said first concentration.12. A method according to claim 9 wherein said catalyst materialcomprises at least an element selected from the group consisting of Ni,Pt, Cu, Ag, Au, In, Sn, Pd, P, As, and Sb.
 13. A method according toclaim 9 wherein said catalyst material comprises at least an elementselected from the group consisting of VIII, IIIb, IVb, and Vb groups inthe periodic table.
 14. A method according to claim 9 wherein saidcatalyst material is formed through a method selected from the groupconsisting of spin coating, dipping, sputtering, vapor phase depositionmethods.
 15. A method according to claim 9 wherein each of said firstand second semiconductor film is a silicon film.
 16. A method accordingto claim 1, wherein crystal growth of the second semiconductor filmproceeds from a portion contacting a surface of the crystallized firstsemiconductor film as a nucleus for the crystal growth.
 17. A methodaccording to claim 1, wherein at least the crystallized secondsemiconductor film acts as an active layer including at least a channelregion of a thin film transistor.
 18. A method according to claim 9,wherein crystal growth of the second semiconductor film proceeds from aportion contacting a surface of the crystallized first semiconductorfilm as a nucleus for the crystal growth.
 19. A method according toclaim 9, wherein at least the crystallized second semiconductor filmacts as an active layer including at least a channel region of a thinfilm transistor.